<!DOCTYPE html>
<style>
  .parent {
    width: 300px;
    padding: 10px;
    border: 1px solid #ccc;
    margin-bottom: 20px;
  }

  .auto {
    width: auto;
    padding: 5px;
    background: lightblue;
  }

  .full {
    width: 100%;
    padding: 5px;
    background: lightgreen;
  }
</style>

<div class="parent">
  <div class="auto">width: autowidth: autowidth: autowidth: autowidth: autowidth: autowidth: auto</div>
</div>

<div class="parent">
  <div class="full">width: 100%width: 100%width: 100%width: 100%width: 100%width: 100%</div>
</div>